Delinking method implemented by UE in wireless communication system, and UE using said method

ABSTRACT

Provided are a delinking method implemented by a remote UE in a wireless communication system, and a device using said method. The method is characterized by: receiving a disconnect message for disconnecting a link between a relay UE and the remote UE; disconnecting the link on the basis of the disconnect message; and not attempting to establish a link between the remote UE and the relay UE in a UE-specified time section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/576,415 filed on Nov. 22, 2017, which is theNational Stage filing under 35 U.S.C. 371 of International ApplicationNo. PCT/KR2016/005592, filed on May 26, 2016, which claims the benefitof U.S. Provisional Applications No. 62/166,143 filed on May 26, 2015,No. 62/166,581 filed on May 26, 2015, and Korean Patent Application No.10-2016-0065069 filed on May 26, 2016, the contents of which are allhereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method of releasing a link, performed by a userequipment (UE) in a wireless communication system, and the UE using themethod.

Related Art

In International Telecommunication Union Radio communication sector(ITU-R), a standardization task for International MobileTelecommunication (IMT)-Advanced, that is, the next-generation mobilecommunication system since the third generation, is in progress.IMT-Advanced sets its goal to support Internet Protocol (IP)-basedmultimedia services at a data transfer rate of 1 Gbps in the stop andslow-speed moving state and of 100 Mbps in the fast-speed moving state.

For example, 3rd Generation Partnership Project (3GPP) is a systemstandard to satisfy the requirements of IMT-Advanced and is preparingfor LTE-Advanced improved from Long Term Evolution (LTE) based onOrthogonal Frequency Division Multiple Access (OFDMA)/SingleCarrier-Frequency Division Multiple Access (SC-FDMA) transmissionschemes. LTE-Advanced is one of strong candidates for IMT-Advanced.

There is a growing interest in a Device-to-Device (D22) technology inwhich devices perform direct communication. In particular, D2D has beenin the spotlight as a communication technology for a public safetynetwork. A commercial communication network is rapidly changing to LTE,but the current public safety network is basically based on the 2Gtechnology in terms of a collision problem with existing communicationstandards and a cost. Such a technology gap and a need for improvedservices are leading to efforts to improve the public safety network.

The public safety network has higher service requirements (reliabilityand security) than the commercial communication network. In particular,if coverage of cellular communication is not affected or available, thepublic safety network also requires direct communication betweendevices, that is, D2D operation.

The D2D operation is also referred to as a Proximity Service (ProSe)operation in a sense that a signal is transmitted/received betweenadjacent devices, and may have various advantages. For example, D2D UEmay perform data communication with a high transfer rate and a lowdelay. Furthermore, in D2D operation, traffic concentrated on a basestation can be distributed. If D2D UE plays the role of a relay, it mayalso play the role of extending coverage of a base station.

The D2D UE may also operate as a UE which plays a role of a relay forconnecting a sidelink and a cellular link. That is, the D2D UE mayoperate as a relay UE. The relay UE may play a role of a relay between aspecific UE and a network. In this case, the specific UE may be referredto as a remote UE.

Meanwhile, after the relay UE and the remote UE establish a connection,it may be necessary to release the connection. For example, there is acase where a quality of service provided by the relay UE becomes worseor the relay UE needs to preferentially provide a relay service for adifferent UE. As such, if there is a need to release a link between therelay UE and the remote UE, it may be necessary to specify which methodwill be used to release the link.

SUMMARY OF THE INVENTION

The present invention provides a method of releasing a link, performedby a user equipment (UE) in a wireless communication system, and the UEusing the method.

In one aspect, provided is a method of releasing a link performed by aremote user equipment (UE) in a wireless communication system. Themethod includes receiving a connection release message for releasing alink between a relay UE and the remote UE and releasing the link on thebasis of the connection release message. The remote UE does not attemptto establish a link for the relay UE in a time duration determined in aUE specific manner.

In the time duration, the remote UE may do not attempt to establish thelink for the relay UE by excluding the relay UE from a candidate relayUE.

The time duration may be a value configured autonomously by the remoteUE.

A connection release accept message may be transmitted to the relay UEin response to the connection release message.

The connection release message may comprise information indicating thatcommunication between the relay UE and the remote UE is no longerallowed.

Upon receiving the connection release message comprising theinformation, it may be not attempted to establish the link for the relayUE during the time duration.

Upon receiving the connection release message comprising theinformation, the remote UE may do not attempt to establish the link forthe relay UE by excluding the relay UE from a candidate relay UE duringthe time duration.

The link between the relay UE and the remote UE may be a sidelink whichuses a PC5 interface.

In another aspect, provided is a user equipment (UE). The UE includes aradio frequency (RF) unit for transmitting and receiving a radio signaland a processor operatively coupled to the RF unit. The processor isconfigured for: receiving a connection release message for releasing alink between a relay UE and the UE and releasing the link on the basisof the connection release message. The processor does not attempt toestablish a link for the relay UE in a time duration determined in a UEspecific manner.

After a remote user equipment (UE) and a relay UE establish a linkconnection, it may be necessary to release the link connection forvarious reasons. The present invention provides a method of releasingthe link connection between the remote UE and the relay UE. Since theremote UE is allowed not to attempt a reconnection during a specifictime with respect to the connection-released relay UE, a ping pongphenomenon can be prevented from occurring between the relay UE and theremote UE.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane.

FIG. 3 is a diagram showing a wireless protocol architecture for acontrol plane.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

FIG. 8 illustrates substates which may be owned by UE in the RRC_IDLEstate and a substate transition process.

FIG. 9 shows a basic structure for ProSe.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

FIG. 13 shows an example of a relay UE.

FIG. 14 shows an example of a relation between a relay UE and a remoteUE.

FIG. 15 shows an example of three steps in which a remote UE selects aspecific UE as a relay UE from candidate relay UEs.

FIG. 16 exemplifies a method of releasing a relay link of a remote UE.

FIG. 17 exemplifies a method of operating a remote UE in a relay linkrelease process in greater detail.

FIG. 18 exemplifies a method of operating a remote UE according to atype of a connection release message.

FIG. 19 exemplifies a method of selecting a relay UE under the controlof a network.

FIG. 20 exemplifies a method of operating a relay UE.

FIG. 21 is a block diagram illustrating a terminal in which anembodiment of the present invention is implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

The E-UTRAN includes at least one base station (BS) 20 which provides acontrol plane and a user plane to a user equipment (UE) 10. The UE 10may be fixed or mobile, and may be referred to as another terminology,such as a mobile station (MS), a user terminal (UT), a subscriberstation (SS), a mobile terminal (MT), a wireless device, etc. The BS 20is generally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC) 30, more specifically, to a mobility management entity (MME)through S1-MME and to a serving gateway (S-GW) through S1-U.

The EPC 30 includes an MME, an S-GW, and a packet data network-gateway(P-GW). The MME has access information of the UE or capabilityinformation of the UE, and such information is generally used formobility management of the UE. The S-GW is a gateway having an E-UTRANas an end point. The P-GW is a gateway having a PDN as an end point.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. Among them, a physical (PHY) layer belonging to the first layerprovides an information transfer service by using a physical channel,and a radio resource control (RRC) layer belonging to the third layerserves to control a radio resource between the UE and the network. Forthis, the RRC layer exchanges an RRC message between the UE and the BS.

FIG. 2 is a diagram showing a wireless protocol architecture for a userplane. FIG. 3 is a diagram showing a wireless protocol architecture fora control plane. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 2 and 3 , a PHY layer provides an upper layer with aninformation transfer service through a physical channel. The PHY layeris connected to a medium access control (MAC) layer which is an upperlayer of the PHY layer through a transport channel. Data is transferredbetween the MAC layer and the PHY layer through the transport channel.The transport channel is classified according to how and with whatcharacteristics data is transferred through a radio interface.

Data is moved between different PHY layers, that is, the PHY layers of atransmitter and a receiver, through a physical channel. The physicalchannel may be modulated according to an Orthogonal Frequency DivisionMultiplexing (OFDM) scheme, and use the time and frequency as radioresources.

The functions of the MAC layer include mapping between a logical channeland a transport channel and multiplexing and demultiplexing to atransport block that is provided through a physical channel on thetransport channel of a MAC Service Data Unit (SDU) that belongs to alogical channel. The MAC layer provides service to a Radio Link Control(RLC) layer through the logical channel.

The functions of the RLC layer include the concatenation, segmentation,and reassembly of an RLC SDU. In order to guarantee various types ofQuality of Service (QoS) required by a Radio Bearer (RB), the RLC layerprovides three types of operation mode: Transparent Mode (TM),Unacknowledged Mode (UM), and Acknowledged Mode (AM). AM RLC provideserror correction through an Automatic Repeat Request (ARQ).

The RRC layer is defined only on the control plane. The RRC layer isrelated to the configuration, reconfiguration, and release of radiobearers, and is responsible for control of logical channels, transportchannels, and PHY channels. An RB means a logical route that is providedby the first layer (PHY layer) and the second layers (MAC layer, the RLClayer, and the PDCP layer) in order to transfer data between UE and anetwork.

The function of a Packet Data Convergence Protocol (PDCP) layer on theuser plane includes the transfer of user data and header compression andciphering. The function of the PDCP layer on the user plane furtherincludes the transfer and encryption/integrity protection of controlplane data.

What an RB is configured means a process of defining the characteristicsof a wireless protocol layer and channels in order to provide specificservice and configuring each detailed parameter and operating method. AnRB can be divided into two types of a Signaling RB (SRB) and a Data RB(DRB). The SRB is used as a passage through which an RRC message istransmitted on the control plane, and the DRB is used as a passagethrough which user data is transmitted on the user plane.

If RRC connection is established between the RRC layer of UE and the RRClayer of an E-UTRAN, the UE is in the RRC connected state. If not, theUE is in the RRC idle state.

A downlink transport channel through which data is transmitted from anetwork to UE includes a broadcast channel (BCH) through which systeminformation is transmitted and a downlink shared channel (SCH) throughwhich user traffic or control messages are transmitted. Traffic or acontrol message for downlink multicast or broadcast service may betransmitted through the downlink SCH, or may be transmitted through anadditional downlink multicast channel (MCH). Meanwhile, an uplinktransport channel through which data is transmitted from UE to a networkincludes a random access channel (RACH) through which an initial controlmessage is transmitted and an uplink shared channel (SCH) through whichuser traffic or control messages are transmitted.

Logical channels that are placed over the transport channel and that aremapped to the transport channel include a broadcast control channel(BCCH), a paging control channel (PCCH), a common control channel(CCCH), a multicast control channel (MCCH), and a multicast trafficchannel (MTCH).

The physical channel includes several OFDM symbols in the time domainand several subcarriers in the frequency domain. One subframe includes aplurality of OFDM symbols in the time domain. An RB is a resourcesallocation unit, and includes a plurality of OFDM symbols and aplurality of subcarriers. Furthermore, each subframe may use specificsubcarriers of specific OFDM symbols (e.g., the first OFDM symbol) ofthe corresponding subframe for a physical downlink control channel(PDCCH), that is, an L1/L2 control channel A Transmission Time Interval(TTI) is a unit time for subframe transmission.

The RRC state of UE and an RRC connection method are described below.

The RRC state means whether or not the RRC layer of UE is logicallyconnected to the RRC layer of the E-UTRAN. A case where the RRC layer ofUE is logically connected to the RRC layer of the E-UTRAN is referred toas an RRC connected state. A case where the RRC layer of UE is notlogically connected to the RRC layer of the E-UTRAN is referred to as anRRC idle state. The E-UTRAN may check the existence of corresponding UEin the RRC connected state in each cell because the UE has RRCconnection, so the UE may be effectively controlled. In contrast, theE-UTRAN is unable to check UE in the RRC idle state, and a Core Network(CN) manages UE in the RRC idle state in each tracking area, that is,the unit of an area greater than a cell. That is, the existence ornon-existence of UE in the RRC idle state is checked only for each largearea. Accordingly, the UE needs to shift to the RRC connected state inorder to be provided with common mobile communication service, such asvoice or data.

When a user first powers UE, the UE first searches for a proper cell andremains in the RRC idle state in the corresponding cell. The UE in theRRC idle state establishes RRC connection with an E-UTRAN through an RRCconnection procedure when it is necessary to set up the RRC connection,and shifts to the RRC connected state. A case where UE in the RRC idlestate needs to set up RRC connection includes several cases. Forexample, the cases may include a need to send uplink data for a reason,such as a call attempt by a user, and to send a response message as aresponse to a paging message received from an E-UTRAN.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

In the NAS layer, in order to manage the mobility of UE, two types ofstates: EPS Mobility Management-REGISTERED (EMM-REGISTERED) andEMM-DEREGISTERED are defined. The two states are applied to UE and theMME. UE is initially in the EMM-DEREGISTERED state. In order to access anetwork, the UE performs a process of registering it with thecorresponding network through an initial attach procedure. If the attachprocedure is successfully performed, the UE and the MME become theEMM-REGISTERED state.

In order to manage signaling connection between UE and the EPC, twotypes of states: an EPS Connection Management (ECM)-IDLE state and anECM-CONNECTED state are defined. The two states are applied to UE andthe MME. When the UE in the ECM-IDLE state establishes RRC connectionwith the E-UTRAN, the UE becomes the ECM-CONNECTED state. The MME in theECM-IDLE state becomes the ECM-CONNECTED state when it establishes S1connection with the E-UTRAN. When the UE is in the ECM-IDLE state, theE-UTRAN does not have information about the context of the UE.Accordingly, the UE in the ECM-IDLE state performs procedures related toUE-based mobility, such as cell selection or cell reselection, without aneed to receive a command from a network. In contrast, when the UE is inthe ECM-CONNECTED state, the mobility of the UE is managed in responseto a command from a network. If the location of the UE in the ECM-IDLEstate is different from a location known to the network, the UE informsthe network of its corresponding location through a tracking area updateprocedure.

System information is described below.

System information includes essential information that needs to be knownby UE in order for the UE to access a BS. Accordingly, the UE needs tohave received all pieces of system information before accessing the BS,and needs to always have the up-to-date system information. Furthermore,the BS periodically transmits the system information because the systeminformation is information that needs to be known by all UEs within onecell. The system information is divided into a Master Information Block(MIB) and a plurality of System Information Blocks (SIBs).

The MIB may include the limited number of parameters which are the mostessential and are most frequently transmitted in order to obtain otherinformation from a cell. UE first discovers an MIB after downlinksynchronization. The MIB may include information, such as a downlinkchannel bandwidth, a PHICH configuration, an SFN supportingsynchronization and operating as a timing reference, and an eNBtransmission antenna configuration. The MIB may be broadcasted on a BCH.

SystemInformationBlockType1 (SIB1) of included SIBs is included in a“SystemInformationBlockType1” message and transmitted. Other SIBs otherthan the SIB1 are included in a system information message andtransmitted. The mapping of the SIBs to the system information messagemay be flexibly configured by a scheduling information list parameterincluded in the SIB 1. In this case, each SIB is included in a singlesystem information message. Only SIBs having the same schedulingrequired value (e.g. period) may be mapped to the same systeminformation message. Furthermore, SystemInformationBlockType2 (SIB2) isalways mapped to a system information message corresponding to the firstentry within the system information message list of a schedulinginformation list. A plurality of system information messages may betransmitted within the same period. The SIB1 and all of the systeminformation messages are transmitted on a DL-SCH.

In addition to broadcast transmission, in the E-UTRAN, the SIB1 may bechannel-dedicated signaling including a parameter set to have the samevalue as an existing set value. In this case, the SIB1 may be includedin an RRC connection re-establishment message and transmitted.

The SIB1 includes information related to UE cell access and defines thescheduling of other SIBs. The SIB1 may include information related tothe PLMN identifiers, Tracking Area Code (TAC), and cell ID of anetwork, a cell barring state indicative of whether a cell is a cell onwhich UE can camp, a required minimum reception level within a cellwhich is used as a cell reselection reference, and the transmission timeand period of other SIBs.

The SIB2 may include radio resource configuration information common toall types of UE. The SIB2 may include information related to an uplinkcarrier frequency and uplink channel bandwidth, an RACH configuration, apage configuration, an uplink power control configuration, a soundingreference signal configuration, a PUCCH configuration supportingACK/NACK transmission, and a PUSCH configuration.

UE may apply a procedure for obtaining system information and fordetecting a change of system information to only a PCell. In an SCell,when the corresponding SCell is added, the E-UTRAN may provide all typesof system information related to an RRC connection state operationthrough dedicated signaling. When system information related to aconfigured SCell is changed, the E-UTRAN may release a considered SCelland add the considered SCell later. This may be performed along with asingle RRC connection re-establishment message. The E-UTRAN may set avalue broadcast within a considered SCell and other parameter valuethrough dedicated signaling.

UE needs to guarantee the validity of a specific type of systeminformation. Such system information is called required systeminformation. The required system information may be defined as follows.

-   -   If UE is in the RRC_IDLE state: the UE needs to have the valid        version of the MIB and the SIB1 in addition to the SIB2 to SIB8.        This may comply with the support of a considered RAT.    -   If UE is in the RRC connection state: the UE needs to have the        valid version of the MIB, SIB1, and SIB2.

In general, the validity of system information may be guaranteed up to amaximum of 3 hours after being obtained.

In general, service that is provided to UE by a network may beclassified into three types as follows. Furthermore, the UE differentlyrecognizes the type of cell depending on what service may be provided tothe UE. In the following description, a service type is first described,and the type of cell is described.

1) Limited service: this service provides emergency calls and anEarthquake and Tsunami Warning System (ETWS), and may be provided by anacceptable cell.

2) Suitable service: this service means public service for common uses,and may be provided by a suitable cell (or a normal cell).

3) Operator service: this service means service for communicationnetwork operators. This cell may be used by only communication networkoperators, but may not be used by common users.

In relation to a service type provided by a cell, the type of cell maybe classified as follows.

1) An acceptable cell: this cell is a cell from which UE may be providedwith limited service. This cell is a cell that has not been barred froma viewpoint of corresponding UE and that satisfies the cell selectioncriterion of the UE.

2) A suitable cell: this cell is a cell from which UE may be providedwith suitable service. This cell satisfies the conditions of anacceptable cell and also satisfies additional conditions. The additionalconditions include that the suitable cell needs to belong to a PublicLand Mobile Network (PLMN) to which corresponding UE may access and thatthe suitable cell is a cell on which the execution of a tracking areaupdate procedure by the UE is not barred. If a corresponding cell is aCSG cell, the cell needs to be a cell to which UE may access as a memberof the CSG.

3) A barred cell: this cell is a cell that broadcasts informationindicative of a barred cell through system information.

4) A reserved cell: this cell is a cell that broadcasts informationindicative of a reserved cell through system information.

FIG. 4 is a flowchart illustrating the operation of UE in the RRC idlestate. FIG. 4 illustrates a procedure in which UE that is initiallypowered on experiences a cell selection process, registers it with anetwork, and then performs cell reselection if necessary.

Referring to FIG. 4 , the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This process is called cell reselectiondifferently from the initial cell selection of the No. 2 process. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 is a flowchart illustrating a process of establishing RRCconnection.

UE sends an RRC connection request message that requests RRC connectionto a network (S510). The network sends an RRC connection establishmentmessage as a response to the RRC connection request (S520). Afterreceiving the RRC connection establishment message, the UE enters RRCconnected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 is a flowchart illustrating an RRC connection reconfigurationprocess. An RRC connection reconfiguration is used to modify RRCconnection. This is used to establish/modify/release RBs, performhandover, and set up/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

Hereinafter, a public land mobile network (PLMN) is described.

The PLMN is a network which is disposed and operated by a mobile networkoperator. Each mobile network operator operates one or more PLMNs. EachPLMN may be identified by a Mobile Country Code (MCC) and a MobileNetwork Code (MNC). PLMN information of a cell is included in systeminformation and broadcasted.

In PLMN selection, cell selection, and cell reselection, various typesof PLMNs may be considered by the terminal.

Home PLMN (HPLMN): PLMN having MCC and MNC matching with MCC and MNC ofa terminal IMSI.

Equivalent HPLMN (EHPLMN): PLMN serving as an equivalent of an HPLMN.

Registered PLMN (RPLMN): PLMN successfully finishing locationregistration.

Equivalent PLMN (EPLMN): PLMN serving as an equivalent of an RPLMN.

Each mobile service consumer subscribes in the HPLMN. When a generalservice is provided to the terminal through the HPLMN or the EHPLMN, theterminal is not in a roaming state. Meanwhile, when the service isprovided to the terminal through a PLMN except for the HPLMN/EHPLMN, theterminal is in the roaming state. In this case, the PLMN refers to aVisited PLMN (VPLMN).

When UE is initially powered on, the UE searches for available PublicLand Mobile Networks (PLMNs) and selects a proper PLMN from which the UEis able to be provided with service. The PLMN is a network that isdeployed or operated by a mobile network operator. Each mobile networkoperator operates one or more PLMNs. Each PLMN may be identified byMobile Country Code (MCC) and Mobile Network Code (MNC). Informationabout the PLMN of a cell is included in system information andbroadcasted. The UE attempts to register it with the selected PLMN. Ifregistration is successful, the selected PLMN becomes a Registered PLMN(RPLMN). The network may signalize a PLMN list to the UE. In this case,PLMNs included in the PLMN list may be considered to be PLMNs, such asRPLMNs. The UE registered with the network needs to be able to be alwaysreachable by the network. If the UE is in the ECM-CONNECTED state(identically the RRC connection state), the network recognizes that theUE is being provided with service. If the UE is in the ECM-IDLE state(identically the RRC idle state), however, the situation of the UE isnot valid in an eNB, but is stored in the MME. In such a case, only theMME is informed of the location of the UE in the ECM-IDLE state throughthe granularity of the list of Tracking Areas (TAs). A single TA isidentified by a Tracking Area Identity (TAI) formed of the identifier ofa PLMN to which the TA belongs and Tracking Area Code (TAC) thatuniquely expresses the TA within the PLMN.

Thereafter, the UE selects a cell that belongs to cells provided by theselected PLMN and that has signal quality and characteristics on whichthe UE is able to be provided with proper service.

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

A method and a procedure of selecting a cell by a terminal in a 3GPP LTEis described with reference to 3GPP TS 36.304 V8.5.0 (2009-03) “UserEquipment (UE) procedures in idle mode (Release 8)”.

A cell selection process is basically divided into two types.

The first is an initial cell selection process. In this process, UE doesnot have preliminary information about a wireless channel. Accordingly,the UE searches for all wireless channels in order to find out a propercell. The UE searches for the strongest cell in each channel Thereafter,if the UE has only to search for a suitable cell that satisfies a cellselection criterion, the UE selects the corresponding cell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection process. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a process, the UEperforms an initial cell selection process.

The cell selection criterion may be defined as below equation 1.Srxlev>0AND Squal>0where:Srxlev=Q _(rxlevmeas)(Q _(rxlevmin) +Q _(rxlevminoffset))−PcompensationSqual=Q _(qualmeas)(Q _(qualmin) +Q _(qualminoffset))  [Equation 1]

Here, the variables in the equation 1 may be defined as below table 1.

TABLE 1 Srxlev Cell selection RX level value (dB) Squal Cell selectionquality value (dB) Q_(rxlevmeas) Measured cell RX level value (RSRP)Q_(qualmeas) Measured cell quality value (RSRQ) Q_(rxlevmin) Minimumrequired RX level in the cell (dBm) Q_(qualmin) Minimum required qualitylevel in the cell (dB) Q_(rxlevminoffset) Offset to the signalledQ_(rxlevmin) taken into account in the Srxlev evaluation as a result ofa periodic search for a higher priority PLMN while camped normally in aVPLMN Q_(qualminoffset) Offset to the signalled Q_(qualmin) taken intoaccount in the Squal evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in a VPLMN Pcompensationmax(P_(EMAX) − P_(PowerClass), 0) (dB) P_(EMAX) Maximum TX power levelan UE may use when transmitting on the uplink in the cell (dBm) definedas P_(EMAX) in [TS 36.101] P_(PowerClass) Maximum RF output power of theUE (dBm) according to the UE power class as defined in [TS 36.101]

Signalled values, i.e., Q_(rxlevminoffset) and Q_(qualminoffset), may beapplied to a case where cell selection is evaluated as a result ofperiodic search for a higher priority PLMN during a UE camps on a normalcell in a VPLMN. During the periodic search for the higher priority PLMNas described above, the UE may perform the cell selection evaluation byusing parameter values stored in other cells of the higher priorityPLMN.

After the UE selects a specific cell through the cell selection process,the intensity or quality of a signal between the UE and a BS may bechanged due to a change in the mobility or wireless environment of theUE. Accordingly, if the quality of the selected cell is deteriorated,the UE may select another cell that provides better quality. If a cellis reselected as described above, the UE selects a cell that providesbetter signal quality than the currently selected cell. Such a processis called cell reselection. In general, a basic object of the cellreselection process is to select a cell that provides UE with the bestquality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection process compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the        same center frequency as that of RAT, such as a cell on which        the UE camps on.    -   Inter-frequency cell reselection: UE reselects a cell having a        different center frequency from that of RAT, such as a cell on        which the UE camps on    -   Inter-RAT cell reselection: UE reselects a cell that uses RAT        different from RAT on which the UE camps

The principle of a cell reselection process is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency.

For the intra-frequency cell reselection or the inter-frequency cellreselection, a network may provide UE with a Neighboring Cell List (NCL)used in cell reselection. The NCL includes a cell-specific parameter(e.g., a cell-specific offset) used in cell reselection.

For the intra-frequency or inter-frequency cell reselection, a networkmay provide UE with a cell reselection black list used in cellreselection. The UE does not perform cell reselection on a cell includedin the black list.

Ranking performed in a cell reselection evaluation process is describedbelow.

A ranking criterion used to apply priority to a cell is defined as inEquation 1.Rs=Qmeas,s+Qhyst,Rn=Qmeas,s−Qoffset  [Equation 2]

In this case, Rs is the ranking criterion of a serving cell, Rn is theranking criterion of a neighbor cell, Qmeas,s is the quality value ofthe serving cell measured by UE, Qmeas,n is the quality value of theneighbor cell measured by UE, Qhyst is the hysteresis value for ranking,and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell.

In accordance with the criterion, it may be checked that the quality ofa cell is the most important criterion in cell reselection. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell reselection.

A Radio Link Failure (RLF) is described below.

UE continues to perform measurements in order to maintain the quality ofa radio link with a serving cell from which the UE receives service. TheUE determines whether or not communication is impossible in a currentsituation due to the deterioration of the quality of the radio link withthe serving cell. If communication is almost impossible because thequality of the serving cell is too low, the UE determines the currentsituation to be an RLF.

If the RLF is determined, the UE abandons maintaining communication withthe current serving cell, selects a new cell through cell selection (orcell reselection) procedure, and attempts RRC connectionre-establishment with the new cell.

In the specification of 3GPP LTE, the following examples are taken ascases where normal communication is impossible.

-   -   A case where UE determines that there is a serious problem in        the quality of a downlink communication link (a case where the        quality of a PCell is determined to be low while performing RLM)        based on the radio quality measured results of the PHY layer of        the UE    -   A case where uplink transmission is problematic because a random        access procedure continues to fail in the MAC sublayer.    -   A case where uplink transmission is problematic because uplink        data transmission continues to fail in the RLC sublayer.    -   A case where handover is determined to have failed.    -   A case where a message received by UE does not pass through an        integrity check.

An RRC connection re-establishment procedure is described in more detailbelow.

FIG. 7 is a diagram illustrating an RRC connection re-establishmentprocedure.

Referring to FIG. 7 , UE stops using all the radio bearers that havebeen configured other than a Signaling Radio Bearer (SRB) #0, andinitializes a variety of kinds of sublayers of an Access Stratum (AS)(S710). Furthermore, the UE configures each sublayer and the PHY layeras a default configuration. In this process, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB 1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 illustrates substates which may be owned by UE in the RRC_IDLEstate and a substate transition process.

Referring to FIG. 8 , UE performs an initial cell selection process(S801). The initial cell selection process may be performed when thereis no cell information stored with respect to a PLMN or if a suitablecell is not discovered.

If a suitable cell is unable to be discovered in the initial cellselection process, the UE transits to any cell selection state (S802).The any cell selection state is the state in which the UE has not campedon a suitable cell and an acceptable cell and is the state in which theUE attempts to discover an acceptable cell of a specific PLMN on whichthe UE may camp. If the UE has not discovered any cell on which it maycamp, the UE continues to stay in the any cell selection state until itdiscovers an acceptable cell.

If a suitable cell is discovered in the initial cell selection process,the UE transits to a normal camp state (S803). The normal camp staterefers to the state in which the UE has camped on the suitable cell. Inthis state, the UE may select and monitor a paging channel based oninformation provided through system information and may perform anevaluation process for cell reselection.

If a cell reselection evaluation process (S804) is caused in the normalcamp state (S803), the UE performs a cell reselection evaluation process(S804). If a suitable cell is discovered in the cell reselectionevaluation process (S804), the UE transits to the normal camp state(S803) again.

If an acceptable cell is discovered in the any cell selection state(S802), the UE transmits to any cell camp state (S805). The any cellcamp state is the state in which the UE has camped on the acceptablecell.

In the any cell camp state (S805), the UE may select and monitor apaging channel based on information provided through system informationand may perform the evaluation process (S806) for cell reselection. Ifan acceptable cell is not discovered in the evaluation process (S806)for cell reselection, the UE transits to the any cell selection state(S802).

Now, a device-to-device (D2D) operation is described. In 3GPP LTE-A, aservice related to the D2D operation is called a proximity service(ProSe). Now, the ProSe is described. Hereinafter, the ProSe is the sameconcept as the D2D operation, and the ProSe and the D2D operation may beused without distinction.

The ProSe includes ProSe direction communication and ProSe directdiscovery. The ProSe direct communication is communication performedbetween two or more proximate UEs. The UEs may perform communication byusing a protocol of a user plane. A ProSe-enabled UE implies a UEsupporting a procedure related to a requirement of the ProSe. Unlessotherwise specified, the ProSe-enabled UE includes both of a publicsafety UE and a non-public safety UE. The public safety UE is a UEsupporting both of a function specified for a public safety and a ProSeprocedure, and the non-public safety UE is a UE supporting the ProSeprocedure and not supporting the function specified for the publicsafety.

ProSe direct discovery is a process for discovering anotherProSe-enabled UE adjacent to ProSe-enabled UE. In this case, only thecapabilities of the two types of ProSe-enabled UE are used. EPC-levelProSe discovery means a process for determining, by an EPC, whether thetwo types of ProSe-enabled UE are in proximity and notifying the twotypes of ProSe-enabled UE of the proximity.

Hereinafter, for convenience, the ProSe direct communication may bereferred to as D2D communication, and the ProSe direct discovery may bereferred to as D2D discovery.

FIG. 9 shows a basic structure for ProSe.

Referring to FIG. 9 , the basic structure for ProSe includes an E-UTRAN,an EPC, a plurality of types of UE including a ProSe applicationprogram, a ProSe application server (a ProSe APP server), and a ProSefunction.

The EPC represents an E-UTRAN core network configuration. The EPC mayinclude an MME, an S-GW, a P-GW, a policy and charging rules function(PCRF), a home subscriber server (HSS) and so on.

The ProSe APP server is a user of a ProSe capability for producing anapplication function. The ProSe APP server may communicate with anapplication program within UE. The application program within UE may usea ProSe capability for producing an application function.

The ProSe function may include at least one of the followings, but isnot necessarily limited thereto.

-   -   Interworking via a reference point toward the 3rd party        applications    -   Authorization and configuration of UE for discovery and direct        communication    -   Enable the functionality of EPC level ProSe discovery    -   ProSe related new subscriber data and handling of data storage,        and also handling of the ProSe identities    -   Security related functionality    -   Provide control towards the EPC for policy related functionality    -   Provide functionality for charging (via or outside of the EPC,        e.g., offline charging)

A reference point and a reference interface in the basic structure forProSe are described below.

-   -   PC1: a reference point between the ProSe application program        within the UE and the ProSe application program within the ProSe        APP server. This is used to define signaling requirements in an        application dimension.    -   PC2: a reference point between the ProSe APP server and the        ProSe function. This is used to define an interaction between        the ProSe APP server and the ProSe function. The update of        application data in the ProSe database of the ProSe function may        be an example of the interaction.    -   PC3: a reference point between the UE and the ProSe function.        This is used to define an interaction between the UE and the        ProSe function. A configuration for ProSe discovery and        communication may be an example of the interaction.    -   PC4: a reference point between the EPC and the ProSe function.        This is used to define an interaction between the EPC and the        ProSe function. The interaction may illustrate the time when a        path for 1:1 communication between types of UE is set up or the        time when ProSe service for real-time session management or        mobility management is authenticated.    -   PC5: a reference point used for using control/user plane for        discovery and communication, relay, and 1:1 communication        between types of UE.    -   PC6: a reference point for using a function, such as ProSe        discovery, between users belonging to different PLMNs.    -   SGi: this may be used to exchange application data and types of        application dimension control information.

<ProSe Direct Communication>

ProSe direct communication is communication mode in which two types ofpublic safety UE can perform direct communication through a PC 5interface. Such communication mode may be supported when UE is suppliedwith services within coverage of an E-UTRAN or when UE deviates fromcoverage of an E-UTRAN.

FIG. 10 shows the deployment examples of types of UE performing ProSedirect communication and cell coverage.

Referring to FIG. 10(a), types of UE A and B may be placed outside cellcoverage. Referring to FIG. 10(b), UE A may be placed within cellcoverage, and UE B may be placed outside cell coverage. Referring toFIG. 10(c), types of UE A and B may be placed within single cellcoverage. Referring to FIG. 10(d), UE A may be placed within coverage ofa first cell, and UE B may be placed within coverage of a second cell.

ProSe direct communication may be performed between types of UE placedat various positions as in FIG. 10 .

Meanwhile, the following IDs may be used in ProSe direct communication.

A source layer-2 ID: this ID identifies the sender of a packet in the PC5 interface.

A destination layer-2 ID: this ID identifies the target of a packet inthe PC 5 interface.

An SA L1 ID: this ID is the ID of scheduling assignment (SA) in the PC 5interface.

FIG. 11 shows a user plane protocol stack for ProSe directcommunication.

Referring to FIG. 11 , the PC 5 interface includes a PDCH, RLC, MAC, andPHY layers.

In ProSe direct communication, HARQ feedback may not be present. An MACheader may include a source layer-2 ID and a destination layer-2 ID.

<Radio Resource Assignment for ProSe Direct Communication>

ProSe-enabled UE may use the following two types of mode for resourceassignment for ProSe direct communication.

1. Mode 1

Mode 1 is mode in which resources for ProSe direct communication arescheduled by an eNB. UE needs to be in the RRC_CONNECTED state in orderto send data in accordance with mode 1. The UE requests a transmissionresource from an eNB. The eNB performs scheduling assignment andschedules resources for sending data. The UE may send a schedulingrequest to the eNB and send a ProSe Buffer Status Report (BSR). The eNBhas data to be subjected to ProSe direct communication by the UE basedon the ProSe BSR and determines that a resource for transmission isrequired.

2. Mode 2

Mode 2 is mode in which UE directly selects a resource. UE directlyselects a resource for ProSe direct communication in a resource pool.The resource pool may be configured by a network or may have beenpreviously determined.

Meanwhile, if UE has a serving cell, that is, if the UE is in theRRC_CONNECTED state with an eNB or is placed in a specific cell in theRRC_IDLE state, the UE is considered to be placed within coverage of theeNB.

If UE is placed outside coverage, only mode 2 may be applied. If the UEis placed within the coverage, the UE may use mode 1 or mode 2 dependingon the configuration of an eNB.

If another exception condition is not present, only when an eNB performsa configuration, UE may change mode from mode 1 to mode 2 or from mode 2to mode 1.

<ProSe Direct Discovery>

ProSe direct discovery refers to a procedure that is used forProSe-enabled UE to discover another ProSe-enabled UE in proximity andis also called D2D direct discovery. In this case, E-UTRA radio signalsthrough the PC 5 interface may be used. Information used in ProSe directdiscovery is hereinafter called discovery information.

FIG. 12 shows the PC 5 interface for D2D direct discovery.

Referring to FIG. 12 , the PC 5 interface includes an MAC layer, a PHYlayer, and a ProSe Protocol layer, that is, a higher layer. The higherlayer (the ProSe Protocol) handles the permission of the announcementand monitoring of discovery information. The contents of the discoveryinformation are transparent to an access stratum (AS). The ProSeProtocol transfers only valid discovery information to the AS forannouncement.

The MAC layer receives discovery information from the higher layer (theProSe Protocol). An IP layer is not used to send discovery information.The MAC layer determines a resource used to announce discoveryinformation received from the higher layer. The MAC layer produces anMAC protocol data unit (PDU) for carrying discovery information andsends the MAC PDU to the physical layer. An MAC header is not added.

In order to announce discovery information, there are two types ofresource assignment.

1. Type 1

As a method in which resources for announcement of discoveredinformation are allocated not specifically to a terminal, a base stationprovides a resource pool configuration for announcement of thediscovered information to terminals. The configuration is included in asystem information block (SIB) to be signaled by a broadcast scheme.Alternatively, the configuration may be provided while being included ina terminal specific RRC message. Alternatively, the configuration may bebroadcast signaling of another layer except for an RRC message orterminal specific signaling.

The terminal autonomously selects the resource from an indicatedresource pool and announces the discovery information by using theselected resource. The terminal may announce the discovery informationthrough an arbitrarily selected resource during each discovery period.

2. Type 2

The type 2 is a method for assigning a resource for announcing discoveryinformation in a UE-specific manner. UE in the RRC_CONNECTED state mayrequest a resource for discovery signal announcement from an eNB throughan RRC signal. The eNB may announce a resource for discovery signalannouncement through an RRC signal. A resource for discovery signalmonitoring may be assigned within a resource pool configured for typesof UE.

An eNB 1) may announce a type 1 resource pool for discovery signalannouncement to UE in the RRC_IDLE state through the SIB. Types of UEwhose ProSe direct discovery has been permitted use the type 1 resourcepool for discovery information announcement in the RRC_IDLE state.Alternatively, the eNB 2) announces that the eNB supports ProSe directdiscovery through the SIB, but may not provide a resource for discoveryinformation announcement. In this case, UE needs to enter theRRC_CONNECTED state for discovery information announcement.

An eNB may configure that UE has to use a type 1 resource pool fordiscovery information announcement or has to use a type 2 resourcethrough an RRC signal in relation to UE in the RRC_CONNECTED state.

Hereinafter, a sidelink may imply a UE-to-UE interface for a D2Dcommunication and a D2D discovery. The sidelink corresponds to theaforementioned PC5 interface. A channel defined/used in the sidelink maybe a physical sidelink control channel (PSCCH), and a control channelfor broadcasting basic system information for D2D communication may be aphysical sidelink broadcast channel (PSBCH). In addition, a channel fortransmitting a D2D discovery signal may be defined as a physicalsidelink discovery channel (PSDCH). A D2D synchronization signal may bereferred to as a sidelink synchronization signal (SLSS) or a D2Dsynchronization signal (D2DSS).

In an LTE-A system (Rel-12, 13, or more), a D2D communication UE isconfigured to transmit the PSBCH and the SLSS together or to transmitthe SLSS. In addition, in the LTE-A system, a sidelink RSRP (S-RSRP) isnewly defined to adjust synchronization with a different UE in the D2Dcommunication. That is, when UEs intend to perform the D2Dcommunication, the D2D communication may be performed in such a mannerthat the S-RSRP is measured to perform the D2D communication byadjusting synchronization only for UEs of which the S-RSRP is greaterthan or equal to a specific value. In this case, the S-RSRP may bemeasured from a demodulation reference signal (DM-RS) on the PSBCH.However, for a D2D relay operation, the S-RSRP may be measured from aDM-RS on the PSDCH. Meanwhile, for the D2D relay operation, sidelinkdiscovery reference signal received power (SD-RSRP) may be used. TheSD-RSRP may be defined as a linear average of power contribution ofresource elements for carrying a DM-RS associated with a PDSCH of whicha cyclic redundancy check (CRC) is authenticated. A reference point ofthe SD-RSRP may be an antenna connection portion of the UE. Whenreception diversity is used by the UE, a value lower than the SD-RSRPdepending on an individual diversity branch may not be reported.

In addition, a UE located out-coverage of a cell may measure theS-RSRP/SD-RSRP or the like on the basis of the SLSS and/or the DM-RS ofthe PSBCH/PSCCH/PSSCH to determine whether the UE will be asynchronization source for performing a D2D relay operation.

Hereinafter, the D2D relay operation is also simply referred to as arelay operation, and a UE for performing the D2D relay operation isreferred to as a relay UE. The relay UE may be located between a firstUE and a second UE to relay a signal between the first and second UEs.Alternatively, the relay UE may be located between a different UE and anetwork (cell/eNB) to relay a signal between the different UE and thenetwork. Hereinafter, it is assumed that the relay UE is a UE forrelaying a signal between the different UE and the network.

FIG. 13 shows an example of a relay UE.

A relay UE 132 is a UE for providing network connectivity to a remote UE133. The relay UE 132 plays a role of relaying a signal between theremote UE 133 and a network 131. The remote UE 133 may be a UE which islocated out-of-coverage of an eNB or has a difficulty in performingdirect communication with the eNB even if it is located in-coverage.

While maintaining not only a link with the eNB but also a link with anormal UE (e.g., remote UE), the relay UE may deliver informationreceived from the eNB to the normal UE or may deliver informationreceived from the normal UE to the eNB. In this case, a link between theeNB and the relay UE may be referred to as a backhaul link, and a linkbetween the relay UE and the remote UE may be referred to as an accesslink. The link between the relay UE and the remote UE may be referred toas a sidelink, a direct link, or a D2D link in a sense that it is aUE-to-UE interface, and may be referred to as a relay link in a sensethat it is associated with a relay of a signal.

FIG. 14 shows an example of a relation between a relay UE and a remoteUE.

In FIG. 14 , a UE1 and a UE3 are out-coverage UEs, a UE2 and a UE4 arein-coverage UEs, and an rUE is a relay UE configured to perform a relayoperation. Herein, the UE2 corresponds to an in-coverage UE as to aneNB2, but may correspond to an out-coverage UE as to an eNB1. As to therUE, the eNB1 may be a serving cell.

The rUE is a UE configured as a rUE by an instruction of the eNB1 or acoordination between rUEs or the like. Since the rUE broadcasts adiscovery signal or the like, neighboring UEs may know in advance apresence of the rUE. For uplink transmission, the rUE may receive a D2Dsignal from a UE (i.e., UE4) in a network of a serving cell, a UE (i.e.,UE2) in a network of a neighboring cell, and out-coverage UEs (i.e.,UE1, UE3).

Hereinafter, a process in which a remote UE selects a relay UE isdescribed in detail. In addition, when the remote UE selects the relayUE, which operation/process will be performed in protocol layers of theremote UE is described.

The process in which the remote UE selects the relay UE may roughlyinclude three steps, and RAN assistant information and controlinformation having a different level may be provided for each step. Theremote UE may be located in-coverage of an eNB or may be locatedout-of-coverage of the eNB. A level controlled by the eNB may beaffected according to where the remote UE is located.

FIG. 15 shows an example of three steps in which a remote UE selects aspecific UE as a relay UE from candidate relay UEs.

Referring to FIG. 15 , a relay UE is configured between a candidaterelay UE and a network (S151). This may be referred to as a step 1:configuration of relay UE.

In order for the candidate relay UE to participate in a discoveryoperation and to perform a relay operation between the remote UE and thenetwork, it may be necessary to authenticate the candidate relay UE as aUE which plays a role of a relay from the remote UE to the network.Accordingly, the candidate relay UE may need to enter an RRC_connectedstate so as to be allowed to operate as a relay UE by the network (eNB).

In addition, there are two types of methods applicable to a discoverytransmitted by the relay UE (this is referred to as a relay discovery).That is, there may be a relay discovery transmission initiated by therelay UE and a relay discovery transmission initiated by the remote UE.Which one will be used between the two types of methods may beconfigured/controlled by the eNB.

That is, in order for the relay UE to participate in the relay discoveryand to play a role of a relay between the remote UE and the network, therelay UE may need to enter the RRC_connected state and to be granted bythe eNB.

Next, the candidate relay UE transmits a relay discovery signal to theremote UE (S152). This may be referred to as a step 2: relay discoveryassisted by network.

In step 2, if the remote UE is located out-of-coverage of a cell, theremote UE evaluates candidate relay UEs. If the remote UE is locatedin-coverage of the cell, selecting of a relay UE from the candidaterelay UEs may be performed by a serving cell of the remote UE on thebasis of a measurement report received from the remote UE or thecandidate relay UEs. Herein, it is assumed that the remote UE is locatedout-of-coverage of the cell, and selecting of the relay UE is performedby the remote UE.

For a selection criterion of the relay UE, a connectivity of thecandidate relay UE (e.g., APN information) and parameters for ameasurement result (e.g., RSRP/RSRQ of a sidelink) may be used. Acriterion of selecting the relay UE by the remote UE includes an upperlayer criterion and a lower layer criterion, which will be describedbelow in detail. For the remote UE located in-coverage of a cell, theeNB may be configured such that the relay discovery is initiated by theremote UE.

When the remote UE selects a specific candidate relay UE as the relayUE, a one-to-one sidelink for a relay is established between thespecific candidate relay UE and the remote UE (S153). This may bereferred to as a step 3: layer-2 link establishment through PC5interface.

In step 3, a unicast connection is established between the remote UE andthe relay UE through the PC5 interface. This process may include anauthentication and security setup process. A security aspect of thisprocess may be handled by SA3. Accordingly, this may be referred to as astep 3: security layer-2 link establishment through PC5 interface.

From an RAN aspect, the following four cases may exist as to mobility ofthe remote UE.

Case 1: When an intra-cell remote UE establishes a connection with arelay UE.

An intra-E-UTRAN access mobility support in an ECM_connected state forUEs may include a handover process and a dual connectivity relatedprocess. In the handover process, a decision/command for allowing a UEto use a radio resource provided by a target eNB is provided by a sourceeNB. Similarly, in the dual connectivity related process, adecision/command for allowing the UE to use a radio resource provided bya secondary eNB (SeNB) is provided by a master eNB (MeNB).

Inter-RAT mobility is controlled by a network, and this control isperformed by the source eNB. That is, a decision/command for allowingthe UE to use a radio resource of a target RAT is performed by thesource eNB.

Therefore, mobility for a UE in an RRC_connected state is based on ahandover depending on a network control under the assistance of the UE.An eNB may control this by using a combination of system information anda dedicated message (RRC connection reconfiguration).

A relay UE for relaying transmission from the remote UE to the networkmay be regarded as another target from a perspective of the remote UE.The source eNB may decide to use a relay UE for performing an inter-RAThandover on the remote UE in the RRC_connected state or for performing arelay from the remote UE to the network, and the source eNB may requirea measurement result for this decision.

A UE located in-coverage of a cell may be in an RRC_idle state or anRRC_connected state according to an activation level of the UE. If theUE is in the RRC_connected state, data can be transmitted/received whilemaintaining service continuity without a data loss through a handoverunder the control of the network. In this aspect, the remote UE in theRRC_connected state may be connected to the relay UE under the controlof the network. Since the UE in the RRC_connected state does nottransmit/receive data, it may be unnecessary to establish a connectionwith the relay UE. Accordingly, the UE in the RRC_idle state may need tofirst enter the RRC_connected state.

When selecting the relay UE located in-coverage of the cell, the eNB mayinvolve in the control in various manners. A level at which the eNBinvolves in the control may be divided into levels 0, 1, 2, and 3.

Level 0: The remote UE may select the relay UE according to animplementation of the remote UE. If data is no longer transmitted in aUu interface between the eNB and the UE, an inactive timer releases theremote UE.

Level 1: The remote UE may select the relay UE on the basis ofparameters included in system information provided from the eNB. If datais no longer transmitted in a Uu interface between the eNB and the UE,an inactive timer releases the remote UE.

Level 2: The remote UE may select the relay UE on the basis ofparameters included in a dedicated signal provided from the eNB. If datais no longer transmitted in a Uu interface between the eNB and the UE,an inactive timer releases the remote UE.

Level 3: The remote UE reports to the eNB a measurement result for acandidate relay UE which satisfies a minimum condition. The eNB selectsthe relay UE from the candidate relay UEs, and hands over the remote UEto the selected relay UE. The handover may be instructed by using an RRCconnection reconfiguration message or an RRC release message.

According to the level 3, since the relay UE is selected by the eNB,there is an advantage in that maximum flexibility and consistency withthe conventional operation can be maintained to the maximum extentpossible.

The minimum criterion may include that: 1) connectivity provided by therelay UE shall satisfy a requirement provided by an upper layer; and 2)a quality of sidelink measurement (e.g., sidelink RSRQ) performed by theremote UE by measuring a discovery signal received from the relay UEshall be greater than or equal to a determined threshold.

Case 2: When a remote UE connected to a relay UE selects and connects adifferent relay UE.

In this case, the remote UE may be a UE located out-of-coverage of thecell. The remote UE may perform a relay UE reselection process. Therelay UE reselection process may include the following three steps.

1. A group of relay UEs (this is referred to as a candidate group)satisfying a minimum condition is maintained. Herein, the minimumcondition may be a condition in which a connectivity and sidelinkmeasurement value exceeds a specific threshold. For this, the remote UEmay use a discovery operation.

2. If the currently connected relay UE does not satisfy the minimumcondition, a reselection process for selecting a different relay UE byexcluding the currently connected relay UE in the candidate group istriggered. A timer and/or a hysteresis may be used before excluding thecurrently connected relay UE from the candidate group.

3. Relay UE reselection may be performed on the basis of any one of thefollowing methods.

a. A remote UE assigns a ranking to candidate relay UEs included in acandidate group to select a candidate relay UE having a highest rankingas a relay UE. The ranking may be assigned on the basis of a sidelinkRSRP measurement value between each candidate relay UE and the remoteUE.

b. The remote UE may select a candidate relay UE randomly selected fromcandidate relay UEs included in the candidate group as the relay UE.

c. The remote UE selects the relay UE from the candidate relay UEsincluded in the candidate group on the basis of a UE implementation.That is, the remote UE selects the relay UE according to animplementation method of the remote UE.

Parameters used in a minimum condition for determining the candidaterelay UE included in the candidate group may be preconfigured or may beconfigured by a network. Among the aforementioned methods a, b, and c,the method a, that is, the methods of assigning rankings to thecandidate relay UEs, has an advantage in that a parameter used forassigning rankings can be preconfigured or can be controlled by thenetwork. Which method will be used from the aforementioned methods a, b,and c may be configured by the network or may be predetermined.

Case 3: When a remote UE connected to a relay UE enters into coverage ofa cell.

In this case, the remote UE performs cell selection for selecting anE-UTRAN cell. According to an EMM state, the UE may trigger an RRCconnection establishment procedure. For example, the RRC connectionestablishment procedure may be triggered to update a tracking area (TA).

When the remote UE connected to the relay UE enters into coverage of acell of a specific eNB from outside the coverage of the cell, upondetecting this, the remote UE changes from a state of receiving aservice by means of the relay UE to a state of receiving a service fromthe specific eNB.

Case 4: When a remote UE located out-of-coverage of a cell is connectedto a relay UE.

In this case, the remote UE may perform an initial relay UE selectionprocess including the following two steps.

1. A group of relay UEs satisfying a minimum condition (e.g., aconnectivity and sidelink measurement value exceeds a specificthreshold) is generated (such a group is referred to as a candidategroup). For this, the remote UE may use a sidelink discovery operation,that is, a procedure in which the remote UE attempts to receive asidelink discovery signal transmitted by the relay UE.

2. The relay UE is selected. The relay UE selection process may be basedon any one of the following methods.

a. A remote UE assigns a ranking to candidate relay UEs included in acandidate group to select a candidate relay UE having a highest rankingas a relay UE. The ranking may be assigned on the basis of a sidelinkRSRP (e.g., SD-RSRP) measurement value between each candidate relay UEand the remote UE.

b. The remote UE may select a candidate relay UE randomly selected fromcandidate relay UEs included in the candidate group as the relay UE.

c. The remote UE selects the relay UE from the candidate relay UEsincluded in the candidate group on the basis of a UE implementation.That is, the remote UE selects the relay UE according to animplementation method of the remote UE.

Parameters used in a minimum condition for determining the candidaterelay UE included in the candidate group may be preconfigured or may beconfigured by a network. Among the aforementioned methods a, b, and c,the method a, that is, the methods of assigning rankings to thecandidate relay UEs, has an advantage in that a parameter used forassigning rankings can be preconfigured or can be controlled by thenetwork. Which method will be used from the aforementioned methods a, b,and c may be configured by the network or may be predetermined.

The remote UE may start a procedure of selecting the relay UE when thefollowing condition is satisfied. Herein, the relay UE implies a UEwhich plays a role of a relay between the remote UE and the network.

When the remote UE acquires a list of candidate relay UEs satisfying anupper layer criterion by monitoring a relay discovery signal andacquires a list of candidate relay UEs satisfying a lower layercriterion, the remote UE may start a procedure of selecting a relay UE.The relay discovery signal may be a discovery signal transmitted by aspecific UE for the purpose of providing a relay operation.

As described above, the relay UE may relay communication between anetwork (e.g., eNB) and the remote UE. The present invention proposes amethod of releasing a pre-selected relay UE, that is, a method ofreleasing a link between the remote UE and the relay UE. In addition, amethod of selecting the relay UE under the control of the network isproposed. In addition, a method of reporting an amount of sidelinktraffic is proposed.

The method of releasing the link between the remote UE and the relay UEmay be classified as follows according to an entity of link releasetriggering.

First, an eNB may instruct a relay UE to release a connection betweenthe relay UE and the remote UE. A sidelink between the relay UE and theremote UE is referred to as a relay link. The eNB may report, to therelay UE, an ID of the remote UE related to a relay link to be released.In addition, the eNB may report, to the remote UE, candidate relay UEsthat can be considered in new relay UE selection/reselection. The eNBmay report that there is a need for UE reselection triggering by meansof a specific remote UE. In this case, the ID of the specific remote UEmay be indicated by the eNB.

The eNB may report that triggering of the relay UE selection/reselectionis necessary by means of all remote UEs connected to the relay UE.

Second, the eNB may instruct the remote UE to release the connectionbetween the relay UE and the remote UE. The eNB may report candidaterelay UEs that can be considered in the new relay UEselection/reselection to the remote UE. The eNB may report thattriggering of the relay UE reselection is necessary. The eNB may reportthat triggering of the relay UE selection/reselection is necessary bymeans of all remote UEs connected to the relay UE.

Third, the relay UE may instruct the remote UE to release the connectionbetween the relay UE and the remote UE. The eNB may report candidaterelay UEs that can be considered in the new relay UEselection/reselection to the remote UE. The relay UE may report thattriggering of the relay UE reselection is necessary. The relay UE mayreport that triggering of the relay UE selection/reselection isnecessary by means of all remote UEs connected to the relay UE.

Finally, the remote UE may trigger the relay link release. The remote UEmay request/instruct the relay UE to release the connection, or mayrequest the eNB to perform a process for releasing the relay link.

If a command for requesting for the release of a connection with acurrent relay UE is received or if it is determined to select a newrelay UE due to deterioration in a quality of a relay service providedby the current relay UE or if it is determined that the quality of therelay service provided by the current relay UE has a problem, the remoteUE does not consider the current relay UE as a candidate UE for therelay UE selection. That is, the current relay UE is excluded fromcandidate UEs for the relay UE selection.

As such, the following content may be considered as a condition ormethod for excluding the current relay UE from the candidate relay UE.

1) It may be explicitly indicated that the current relay UE will beexcluded according to a relay link release command. The relay linkrelease command may be provided by the network or the relay UE.

2) Upon receiving a relay link release command not explicitly indicatingthat the current relay UE will be excluded, the remote UE may excludethe current relay UE from the candidate relay UE. For example, ifinformation indicating an ID of a different UE other than the currentrelay UE is included in the relay link release command, the remote UEautonomously excludes the current relay UE from the candidate relay UE.

For another example, when a message for triggering relay UE reselectionis included in the relay link release command, the remote UEautonomously excludes the current relay UE from the candidate relay UE.

Meanwhile, a time at which the current relay UE is excluded from thecandidate relay UE may be defined. The eNB may report a time duration inwhich the remote UE excludes the current relay UE from the candidaterelay UE. In this case, the remote UE may exclude the current relay UEfrom the candidate relay UE from when a message including a timeduration value is received from the eNB. Alternatively, upon receiving amessage indicating the release of the relay UE, the remote UE mayautonomously exclude the relay UE from the candidate relay UE in aspecific time duration. That is, the relay UE may be excluded from thecandidate relay UE in a time duration determined in a UE specificmanner.

FIG. 16 exemplifies a method of releasing a relay link of a remote UE.

Referring to FIG. 16 , it is a state where the relay link is establishedbetween the remote UE and a relay UE (S160). In this case, for variousreasons, the relay UE may transmit a direct connection release message(this is referred to as ‘DIRECT_COMMUNICATION_RELEASE’) indicating therelease of the relay link to the remote UE (S161). A message formanaging such as a sidelink may be transmitted by using a PC5 signalingprotocol.

The relay UE may start a connection release process upon receiving arelay link release with respect to the remote UE from an upper layer ofthe relay UE. That is, a connection release message is generated.

In this case, the connection release message may include a cause valueindicating a cause of a connection release, and the cause value mayinclude any one of values of the following table.

TABLE 2 Cause value Meaning #1 Direct communication with remote UE is nolonger necessary #2 Direct communication with remote UE is no longerallowed #3 Direct communication is no longer available

The relay UE generates a connection release message, and thereaftertransmits the message by delivering it to a lower layer.

Upon receiving the connection release message, the remote UE stops arunning timer, and stops relay data transmission/reception with respectto the relay UE (S162). The remote UE stops the relay datatransmission/reception, but maintains a PC5 signaling protocol fortransmission of a response for the connection release message.

If the cause value included in the connection release message indicates“direct communication with remote UE is no longer allowed” (cause value#2), the remote UE does not attempt to start a direct link setup for therelay UE during at least a specific time T. For example, the remote UEmay not perform the link setup attempt for the relay UE by excluding therelay UE which has transmitted the connection release message from acandidate relay UE during the specific time. The time T may be set bythe network or the relay UE, or may be a predetermined value or a valuedetermined by the remote UE. The specific time T may be a time durationdetermined in a UE-specific manner. In other words, it may be a valuedetermined according to a UE implementation, and in this aspect, may beregarded as a value autonomously determined by the UE. Since the remoteUE does not attempt to start the direct link setup for the relay UEduring at least the specific time T, a ping pong phenomenon can beprevented from occurring. That is, it is possible to prevent the pingpong phenomenon in which, for example, the relay UE is changed from afirst relay UE to a second relay UE, and then the relay UE is changedagain from the second relay UE to the first relay UE.

The remote UE transmits to the relay UE a direct connection releaseaccept message (this may be referred to as‘DIRECT_COMMUNICATION_RELEASE_ACCEPT’) (S163). After transmitting theconnection release accept message, the remote UE removes a contextrelated to a link with the relay UE, and does not transmit/receive anymessage by using a PC5 signaling protocol message through this link.This means that the remote UE discards a MAC PDU in which a UE ID of therelay UE is included as a source ID, instead of delivering it to anupper layer when the MAC PDU is received through a sidelink.

The relay UE may start a timer while transmitting the connection releasemessage, and if the connection release accept message is not receiveduntil the timer expires, may repeat a process of restarting the timer bya predefined maximum number of times while retransmitting the connectionrelease message.

If there is no response from the remote UE until it reaches thepredetermined maximum number of times, the relay UE releases a directlink for the remote UE and does no longer transmit/receive any messagethrough this link. The maximum number of times may be determinedaccording to the UE.

Meanwhile, although an example in which the relay UE starts a relay linkrelease is described in FIG. 16 , the present invention is not limitedthereto. That is, the relay link release may be started by the remoteUE, and in this case, the remote UE may transmit the connection releasemessage to the relay UE. In addition, the relay UE may transmit theconnection release accept message to the remote UE.

FIG. 17 exemplifies a method of operating a remote UE in a relay linkrelease process in greater detail.

Referring to FIG. 17 , the relay UE transmits a connection releasemessage to the remote UE (S171). In this case, a cause value included inthe connection release message may indicate that “direct communicationwith the remote UE is no longer allowed”.

In this case, the remote UE may not attempt a link setup for the relayUE during a time T (S172). The time T may be set by a network or therelay UE, and may be a predetermined value or a value defined by theremote UE.

Meanwhile, the relay UE starts a timer if the cause value included inthe connection release message is “direct communication with the remoteUE is no longer allowed”, and retransmits the connection release messageby a specific number of times if a connection release accept message isnot received before the timer expires. On the other hand, if the causevalue included in the connection release message is “direct connectionis no longer available”, a relay link may be immediately releasedwithout having to start the timer.

FIG. 18 exemplifies a method of operating a remote UE according to atype of a connection release message. That is, the remote UE forperforming a different operation according to a cause value included inthe connection release message is exemplified in FIG. 18 .

Referring to FIG. 18 , while receiving a relay service (relay service)from a relay UE 1 (S2010), the remote UE may receive the connectionrelease message (S2020). The connection release message may have a value‘a’ as the cause value.

In this case, the remote UE may receive signals from the relay UEs 1 and2 (S2031, S2032), and may select the relay UE in a state where the relayUE 1 is included in a candidate relay UE (S2030).

If the relay UE 1 is selected as the relay UE, the remote UE transmits aconnection request message to the relay UE 1 (S2040), receives aconnection accept message (S2050), and thereafter receives a relayservice from the relay UE 1 (S2060).

The remote UE may receive again the connection release message from therelay UE 1 (S2070). In this case, the connection release message mayhave a value ‘b’ as the cause value. In this case, the remote UE mayreceive signals from the relay UEs 1 and 2 (S2081, S2082), and mayselect the relay UE in a state where the relay UE 1 is excluded from thecandidate relay UE (S2080).

If the relay UE 2 is selected as the relay UE, the remote UE transmits aconnection request message to the relay UE 2 (S2090), receives aconnection accept message (S2100), and thereafter receives a relayservice from the relay UE 2 (S2110). The cause values a and b may be oneof values of Table 2, or may be other different values. For example, thecause value a may be a value capable of indicating an operation forincluding a UE which has transmitted the connection release message whenthe relay UE is selected, and the cause value b may be a value capableof indicating an operation for excluding the UE which has transmittedthe connection release message when the relay UE is selected.

Meanwhile, if a command for requesting for the release of a connectionwith a current relay UE is received or if it is determined to select anew relay UE due to deterioration in quality of a relay service providedby the current relay UE or if it is determined that the quality of therelay service provided by the current relay UE has a problem, the remoteUE may apply a bias to the current relay UE. Herein, the bias may be avalue for decreasing a probability that the current relay UE is selectedagain in a relay UE selection/reselection process. When the applying ofthe bias is applied to the current relay UE, a probability that thecurrent relay UE is continuously selected is decreased, and aprobability that another relay UE is selected as a new relay UE isincreased. When the bias is applied, as a result, the current relay UEmay be excluded from a candidate relay UE. A method of applying the biasincludes a method of performing the procedure of selecting/reselectingthe relay UE from other candidate relay UEs by excluding the currentrelay UE from the candidate relay UE.

For example, the remote UE may select the relay UE on the basis of aquality in a link between the relay UE and an eNB and/or a sidelinkbetween the relay UE and the remote UE. In this case, the remote UE mayapply a negative value as a bias for a link related to the current relayUE, and thus may regard that a quality of a link between the currentrelay UE and the eNB or between the current relay UE and the remote UEis not good.

Regarding the bias, the remote UE may autonomously apply the bias byregarding that: 1) the bias is applied directly by a relay link releasecommand; or 2) the basis is applied when the relay link release commandincludes information indicating an ID of a different relay UE or amessage for triggering relay UE reselection even if the relay connectionreject command does not explicitly indicate to apply the bias.

Meanwhile, a time at which the bias is applied may also be defined. Theremote UE may receive the time, at which the bias is applied, fromdirectly the eNB or from the relay UE. The eNB may report a timeduration in which the current relay UE is excluded from the candidaterelay UE to the remote UE located in-coverage. Alternatively, when theeNB designates the time information to the relay UE for the remote UElocated out-of-coverage, the relay UE may transmit this information tothe remote UE. The remote UE may apply the bias from when a messageincluding a time duration value is received from the eNB. For example,the current relay UE may be excluded from the candidate relay UE.

Alternatively, the UE may autonomously apply the bias to the currentrelay UE in a specific time duration from when a relay link releasemessage is received.

Meanwhile, although the remote UE requests a first UE to provide therelay service, the request may be rejected. For example, a relayoperation of the first UE may be rejected by the network or may berejected by the decision of the relay UE. In this case, the remote UEmay apply a bias to the first UE to decrease a probability that thefirst UE is selected in a relay UE selection/reselection processperformed at a later time. For example, if the remote UE selects therelay UE on the basis of a quality in a link between the relay UE andthe eNB and/or a sidelink between the relay UE and the remote UE, theremote UE may apply a negative value as a bias for a link related to thefirst UE and thus may regard that a quality of a link between the firstUE and the eNB or between the first UE and the remote UE is not good,thereby decreasing a probability that the first UE is reselected as therelay UE.

In this aspect, the bias value may be a sort of an offset value. Theoffset value may be a predetermined or configured value or may be avalue signaled by the network. The predetermined or configured value maybe applied to out-of-coverage of a cell. If the offset value is signaledby the network, the signaled value may be preferentially applied insteadof the predetermined or configured value. If the offset value isconfigured by the network, the UE may regard that the offset value isvalid during a maximum time determined after moving to theout-of-coverage of the cell.

Regarding the bias or the offset, the remote UE may autonomously applythe bias (or offset) by regarding that: 1) the bias is applied directlyby a relay connection reject command; or 2) the basis is applied whenthe relay connection reject command includes information indicating anID of a different relay UE or a message for triggering a relay UEreselection even if the relay connection reject command does notexplicitly indicate to apply the bias.

Meanwhile, a time at which the bias or the offset is applied may also bedefined. The eNB may report a time duration in which the remote UEexcludes the current relay UE from the candidate relay UE. In this case,the remote UE may exclude the current relay UE from the candidate relayUE from when a message including a time duration value is received fromthe eNB. Alternatively, the UE may autonomously apply the bias to thecurrent relay UE in a specific time duration from when the relayconnection reject command is received.

Meanwhile, even if the UE is excluded from the candidate relay UE in therelay UE selection/reselection process, it may be necessary to includethe UE again in the candidate relay UE (or to increase a probability ofselecting it or to increase a priority thereof). For example, if thecurrent relay UE is excluded from the candidate relay UE but there isnot even one different UE satisfying a relay UE selection/reselectionrequirement, it may be necessary to include the current relay UE againin the candidate relay UE. That is, the remote UE regards the currentrelay UE as the candidate relay UE. If the remote UE requests the relayUE which must be excluded from the candidate relay UE to configure arelay service/1:1 link, information (e.g., indicator) indicating such anexceptional situation may be included in a request message. In thiscase, the relay UE may provide a limited relay service to the remote UE.Herein, the limited relay service is that the relay service is providedonly for some of services desired by the remote UE or a limited qualityof service (QoS) is provided or the QoS is not ensured for the providedrelay service.

Hereinafter, a method of selecting a relay UE under the control of anetwork is described.

The network may allow the remote UE to select the relay UE by using anyone of: 1) a mode in which the remote UE autonomously selects the relayUE; and 2) a mode in which the relay UE is selected under the control ofthe network.

The network may report which method will be used between theaforementioned two types of relay UE selection methods using an upperlayer signal such as an RRC signal.

First, an operation is described for a case where the network isconfigured to allow the remote UE to autonomously select the relay UE.The network may allow the remote UE to indicate direct setup informationthrough the upper layer signal or may allow the relay UE to relay thesetup information. Meanwhile, in order for the network to control arelay UE selection of the remote UE, if information is collected fromthe remote UE, the information may be directly received from the remoteUE or may be received through a relay of the relay UE.

The remote UE may have a criterion for selecting the relay UE. Thecriterion may be predetermined in the remote UE or may be determined bythe network.

If a UE satisfying the aforementioned criterion is found by searchingfor the UE, the remote UE selects/reselects a corresponding UE as therelay UE.

The remote UE autonomously starts a process for establishing aconnection with the relay UE. The process may include a security setupbetween the remote UE and the relay UE.

The network may report a list of candidate relay UEs to the remote UE.For example, if the candidate relay UEs are identified on the basis of aUE ID, the network may report the UE ID of the candidate relay UEs tothe remote UE. The list may be broadcast or may be provided through adedicated signal. When the list is provided, the remote UE may selectthe relay UE only from UEs included in the list.

The candidate relay UE may report its ID when a D2D discovery signal istransmitted (announced).

FIG. 19 exemplifies a method of selecting a relay UE under the controlof a network.

Referring to FIG. 19 , the network transmits a configuration of a relayUE selection method to a remote UE (S181). It may be reported to theremote UE that the network selects the relay UE for the remote UE. Thatis, it may be reported to the remote UE that the relay UE is selectedunder the control of the network.

The remote UE measures a link quality with respect to each of candidaterelay UEs (S182), and reports candidate relay UEs of which a linkquality is greater than or equal to a threshold to the network (S183).

A measurement report for reporting the candidate relay UEs for a relayservice may be configured for the remote UE. The configuration of themeasurement report may include a minimum sidelink (relay link) qualitythreshold. When the threshold is configured, the remote UE may reportonly candidate relay UEs of which a quality is greater than or equal tothe threshold.

When the candidate relay UEs are reported from the remote UE, an eNBselects a specific candidate relay UE from the candidate relay UEs(S184), and reports the specific candidate relay UE to the remote UE(S185). That is, the network may instruct the remote UE to select thespecific candidate UE as the relay UE. In this case, the eNB may reportan ID of the specific candidate relay UE to the remote UE.

When the eNB instructs the remote UE to select the specific candidaterelay UE as the relay UE, a single or a plurality of specific candidaterelay UEs may be present. If the number of the specific candidate relayUEs is one, the remote UE performs a connection establishment processfor the relay service with respect to the single specific candidaterelay UE. The connection establishment process may include a securityconfiguration with respect to the remote UE and the specific candidaterelay UE.

Meanwhile, the eNB may provide the remote UE with a list for reportingthe plurality of candidate relay UEs. For example, the eNB may reportIDs of the plurality of candidate relay UEs to the remote UE. This listmay be provided through a dedicated signal for the remote UE. If thelist is provided, the remote UE may select the relay UE only fromcandidate relay UEs included in the list.

The remote UE selects a specific candidate relay UE as the relay UE(S186), and establishes a connection for a relay service. The connectionestablishment process may include a security setup between the remote UEand the selected candidate relay UE. The candidate relay UEs included inthe list may report their IDs when a discovery signal is transmitted.

If it is recognized/detected that the remote UE is locatedout-of-coverage of a cell on a carrier frequency at which a sidelinkoperation is performed, the remote UE may change an operation methodfrom a method of selecting the relay UE under the control of the networkto a method of selecting the relay UE autonomously by the remote UE. Onthe contrary, if it is recognized/detected that the remote UE is locatedin-coverage of the cell on the carrier frequency at which the sidelinkoperation is performed, the remote UE may change the operation methodfrom the method of selecting the relay UE autonomously by the remote UEto the method of selecting the relay UE under the control of thenetwork.

Meanwhile, a hysteresis may be necessary when the relay UE isreselected. When the remote UE performs the process of selecting therelay UE, the remote UE may apply the hysteresis for a link quality ofthe current relay UE. This is to prevent the ping pong phenomenon inwhich, for example, the relay UE is changed from a first relay UE to asecond relay UE, and then the relay UE is changed again from the secondrelay UE to the first relay UE.

If a newly selected candidate relay UE does not show a better linkquality greater than or equal to a specific value in comparison with thecurrent relay UE, the remote UE may regard that a reselection criterionis not satisfied for the candidate relay UE.

Meanwhile, the relay UE may provide the relay service between the remoteUE and the network. However, the relay UE may perform communication withthe network separately from the relay service for the remote UE.

In this case, the relay UE may measure a traffic amount in a sidelinkfor the relay service and may report this to the network.

FIG. 20 exemplifies a method of operating a relay UE.

Referring to FIG. 20 , the relay UE measures traffic by distinguishingtraffic for a relay and relay UE specific traffic (S1910). For example,the relay UE may report a traffic amount by separatelymeasuring/calculating a traffic amount for sidelink transmission for arelay service and a traffic amount for sidelink reception for a relayservice.

The relay UE reports the traffic amount for the relay to an eNB (S1920).

The relay UE may accumulatively calculate a size of a PDCP service dataunit (SDU) delivered through the sidelink for the relay service. Therelay UE may calculate an average traffic amount by dividing anaccumulated traffic amount for the sidelink on a time basis.

The relay UE may accumulatively calculate a size of a MAC protocol dataunit (PDU) delivered through the sidelink for the relay service. Therelay UE may calculate an average traffic amount by dividing anaccumulated traffic amount for the sidelink on a time basis.

The relay UE may acquire and add a traffic amount for the relay servicefor all remote UEs connected to the relay UE, and thereafter may reportthis to the network.

The relay UE may calculate an accumulated traffic amount or an averagetraffic amount for each priority of a sidelink logical channelcorresponding to a sidelink radio bearer. Herein, the priority of thesidelink logical channel may be a ProSe per packet priority (PPPP) valueor a priority value related to the PPPP.

Alternatively, the relay UE may acquire a traffic amount for the relayservice for each remote UE connected to the relay UE and may report thisto the network. In this case, the relay UE may report together aspecific UE for which the traffic amount is reported. For example, therelay UE may report together an ID of a remote UE and a traffic amountfor the remote UE. In a handover preparation process for the remote UE,traffic information of the remote UE may be delivered to a target eNB.

The report may be performed periodically or may be performed when aspecific event is satisfied. The specific event may be an event in whicha measured sidelink traffic amount exceeds a threshold.

In a handover preparation process of the relay UE, traffic informationreported by the relay UE may be delivered to the target eNB.

The remote UE may report that it is interested in the relay service.When the remote UE desires to receive the relay service, if the remoteUE exists in coverage of a network, the remote UE may report that it isinterested in receiving the relay service to the network.

When a message for reporting that it is interested in the relay serviceis received from the remote UE, the network may provide a relayconnection management command with a minimum interruption even if SIBhas a transmission resource pool and reception resource pool for asidelink operation by maintaining the remote UE to an RRC_connectedstate.

The UE may report a traffic amount by measuring/calculating a sum of atraffic amount for sidelink transmission for a relay service and atraffic amount for sidelink reception for the relay service.

FIG. 21 is a block diagram illustrating a terminal in which anembodiment of the present invention is implemented.

Referring to FIG. 21 , a terminal 1100 includes a processor 1100, amemory 1120, and a radio frequency (RF) unit 1130. The processor 1110implements a function, a process, and/or a method which are proposed.The RF unit 1130 is connected to the processor 1110 and sends andreceives radio signals.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

What is claimed is:
 1. A method for releasing a link between a relayuser equipment (UE) and a remote UE in a wireless communication system,the method performed by the remote UE and comprising: receiving, fromthe relay UE, a connection release message for releasing the link,wherein the connection release message includes one release reason of aplurality of release reasons, wherein the plurality of release reasonscomprise i) ‘direct communication to the remote UE no longer needed’,ii) ‘direct communication with the remote UE is no longer allowed’ andiii) ‘direct communication is not available anymore;’ releasing the linkbased on the connection release message; and triggering a relayreselection based on the connection release message, wherein the remoteUE selects a new relay UE among candidate relay UEs during the relayreselection, wherein based on the one release reason being the ii)‘direct communication with the remote UE is no longer allowed’ otherthan the i) ‘direct communication to the remote UE no longer needed’ andthe iii) ‘direct communication is not available any more’ among theplurality of release reasons, the remote UE i) shall not attempt tostart direct link setup with the relay UE at least for a time durationand ii) shall exclude the relay UE from the candidate relay UEs duringthe relay reselection, and wherein the time duration is autonomouslydetermined by the remote UE.
 2. A remote user equipment (UE) which isconnected to a relay UE, comprising: a transceiver which transmits orreceives a radio signal; and a processor operatively connected with thetransceiver, wherein the processor is configured to: receive, from therelay UE, a connection release message for releasing a link between theUE and the relay UE, wherein the connection release message includes onerelease reason of a plurality of release reasons, wherein the pluralityof release reasons comprise i) ‘direct communication to the remote UE nolonger needed’, ii) ‘direct communication with the remote UE is nolonger allowed’ and iii) ‘direct communication is not availableanymore;’ release the link based on the connection release message; andtrigger a relay reselection based on the connection release message,wherein the remote UE selects a new relay UE among candidate relay UEsduring the relay reselection, wherein based on the one release reasonbeing the ii) ‘direct communication with the remote UE is no longerallowed’ other than the i) ‘direct communication to the remote UE nolonger needed’ and the iii) ‘direct communication is not available anymore’ among the plurality of release reasons, the remote UE i) shall notattempt to start direct link setup with the relay UE at least for a timeduration and ii) shall exclude the relay UE from the candidate relay UEsduring the relay reselection, and wherein the time duration isautonomously determined by the remote UE.